Everything I ship is declared honestly, and I handle the paperwork on the US export side.
Duties and customs charges in your country, if any are levied, are billed to you at cost on the shipping invoice. They are not inside my 15% and they are not inside the cargo rate. I do not know the figure until customs sets it, so where I expect a charge (high-value electronics, luxury items, anything over NAFDAC thresholds) I tell you before I ship rather than after.
If customs holds your parcel for inspection, I coordinate directly with them and with you. This adds 3–5 days but is normal for items over about $500.